Space technology plays an increasingly important role in our daily life. However, spacecraft is typically hard or expensive to reach for service after launch. Therefore, components, sub-assemblies and systems for space applications require special design and rigorous test in order to perform over space environment, especially irradiation.
Due to the advantages of light weight, compact size, broad bandwidth, and resistance of electromagnetic interference, fiber optics systems and assemblies have been developed and employed for space and nuclear facilities applications including communication, sensing, navigations and etc. However, it has been known that the performance of an optical-fiber-based device would be severely degraded by the RIA (Radiation Induced Attenuation). The RIA could cause severe output power loss of the optical-fiber-based device, and hence greatly limits its operating life. To decrease RIA, an effective method needs to be developed.
In the prior art, some methods have been reported to reduce the RIA in passive single-mode fibers (no rare-earth dopant) through thermal annealing (U.S. Pat. No. 4,229,069), and hydrogen pre-loading approaches (U.S. Pat. No. 6,130,981). However, the thermal annealing approach needs to raise temperature up to 200° C. to 300° C. to decrease RIA effectively. Such a high temperature environment could damage or degrade the device and its associate assembly. On the other hand, the hydrogen pre-loading method needs a hermetic coating to avoid out-diffusion of hydrogen, and the fabrication process of a hermetic coating might be complicated and expensive.
In addition, prior to the present invention, photo-annealing method had been reported, with a limited success, to reduce the RIA in both passive single-mode fibers such as a pure-core fiber and a Ge-doped fiber (U.S. Pat. No. 4,232,228), and an active Er-doped fiber. Moreover, the prior photo-annealing approach for active Er-doped fiber employed only 980-nm light source.
According to the present invention, applicants have departed from the conventional wisdom, and had conceived and implemented a photo-annealing apparatus to include at least a shorter wavelength photo-annealing spectral content, which is relative to that of a pump light source, for effectively photo-annealing the rare-earth-doped fiber element. The photo-annealing by shorter wavelength light results in a fast and nearly complete recovery of RIA of the rare-earth-doped optical fiber element in the wavelength range from 900 nm to 1700 nm. Such a fast and nearly complete RIA recovery is unprecedented in open literature. The invention is briefly described as follows.